Absorptiometric Method for Determination of Cesium WILLIAM G. KROCHTA with M. G. MELLON Purdue University, Lafayette, Ind.
b A new method i s described for the indirect determination of cesium b y precipitation with excess tungstosilicic acid, with subsequent development of a heteropoly blue color from the excess precipitant. The effects of several variables on the precipitation reaction and the color development were determined and optimum conditions were ascertained. Conformity to Beer’s law was found for concentrations between 0.1 33 and 0.931 mg. of cesium per ml. The determination may b e performed in the presence of limited amounts of potassium and rubidium.
A
of methods have been reported in recent years for detcrmining small quantities of cesium (less than 1 mg.) but only a fen were absorptiometric methods (6, 7 ) . As cesium is commonly encountered only in small amounts, the absorptiometric approach is especially appropriate as a means of determination. Because of the cloce similarity in the chemical hehavior of potassium. rubidium, and cesium, practically all of the proposed absorptiometric methods for cesium are also applicable for the other tivo elenleiits ( I ) . Consequently, the presence of one seriously interferes with the determination of the other. Methods which involve precipitating cesium as the picrate, chloroplatinatP. or dipicrylaminate. dissolving the colored precipitate, and measuring the color intensity of the resultant solution, are also applicable to the determination of rubidium and potassium (6, 7 ) . Various methods devised for determination of potassium have been suggested as applicable to rubidium and cesium (1.10). Thus, Langham ( I ) proposed the precipitation of potassium disilver-cobaltinitrite. dissolution, diazotization of sulfanilic acid with the free nitrite ion. and then coupling with 1naphthylamine. Although sensitive, this method is time-consuming. Short ( I ) described the use of potassium iodide after precipitation of the chloroplatinate. The liberated iodine was measured absorptiometrically. colorimetric spot test for cesium was suggested by Burkser and Kutschment ( 4 ) . -4 mixture of gold and platinum hroniideq forms a deep black precipitate with a cesium salt. The amount of (&uni present can be deterNTXIBCR
mined by comparison of the sample with a series of standards. Rubidium interferes to some extent, whereas potassium does not. Burkser and Feldman ( 3 ) used molybdosilicic acid to determine cesium. The precipitate of cesium molybdosilicate was separated and dissolved, and the molybdosilicate was reduced to a heteropoly blue. Potassium and limited amounts of rubidium can be present. Another heteropoly compound, tungstosilicic acid, was used as a precipitant for cesium (8, 9, 1 1 ) . O’Leary and Papish (11) recommended it for the quantitative separation of cesium from rubidium, but because of the uncertain hydration of cesium tungstosilicate, cesium must be reprecipitated as the chloroplatinate before being weighed. I n determining cesium radiometrically, Kahn (9) separated cesium by precipitation with tungstosilicic acid. The purpose of this investigation was to develop a rapid indirect absorptiometric method for small quantities of cesium by utilizing its selective and quantitative precipitation vcith tungstosilicic acid. As no colored products were involved, it was necessary to produce a color by the reduction of the heteropoly acid to a heteropoly blue. Unlike the usual absorptiometric methods involving a preliminary separation of the desired constituent by precipitation, the color m s not developed from further treatment of the precipitate, but rather from treatment of the supernatant liquid. The intensity of the color developed from the unreacted known excess of tungstosilicic acid in the supernatant liquid should be inversely proportional to the amount of cesium precipitated. EXPERIMENTAL WORK
Apparatus and Reagents. Absorption d a t a were obtained on two different instruments. I n the preliminary work, n h e n it n a s necessary t o scan t h e entire visible spectrum, a Cary Model 10-11 31 recording spectrophotometer n a s used. All calibration curves, interference effects, and stability and precision studies were obtained with a Beckman Model B spectrophotometer. Matched 1-em. quartz absorption cells were used with both instruments. The absorptiometer preferably should be capable of measurement of 725 mp, although one operating just under
700 m,u \vi11 serve with somewhat less sensitivity. All p H measurements \\-ere made on a Beckman Model G glass electrode p H meter. Microburets were used to dispense reproducibly definite small volumes of reagents. A centrifuge, operating a t 1750 r.p.ni., was used to hasten settling of the precipitate. It was equipped to hold 15ml. tubes. Tungstosilicic acid, H4[Si(KsOl&],. 24H20 (Fisher Scientific Co.) was purified by repeated recrystallizations from water (If), although the presence of a small amount of tungstic acid impuritx is compensated for by the calibration curve. The tungstosilicic acid solution (approximately 5%) was prepared by dissolving 5 0 grams of purified tungstosilicic acid in 95 ml. of 6 S hydrochloric acid. A large amount of tungstosilicic acid reagent should be prepared, so that the same solution may be used throughout a series of determinations. If it is necessary to prepare a fresh solution, a new calibration curve should also be prepared. The reducing solution (approximately 0.2%) was prepared b y adding 1 ml. of a 20% solution of titanium trichloride (Fisher Scientific Co.) to a 100-nil volumetric flask containing 1 ml. of concentrated hydrochloric acid. The solution mas heated and then diluted to volume with distilled water (13). As it was stable for only approximately 1 hour when exposed to air, it was stored under a carbon dioxide atmosphere. A solution stored in this manner was stable for a t least 3 days. Cesium chloride (CsC1, Fairmount Chemical Co., Inc ) was recrystallized from water. Only a slight trace of potassium and rubidium was detected on a n emission spectrograph using a n arc excitation source. A stock solution containing 1.33 mg. of cesium per ml. n-as prepared by placing 0.1684 gram of cesium chloride in a 100-ml. volumetric flask, and dissolving and diluting it to volume with 6 S hydrochloric acid. All other reagents were of reagent grade and required no further purification. Solutions were stored in polyethylene containers. Precipitation Reaction. T h e reaction between cesium and tungstosilicic acid yields a white, crystalline precipitate in 6 S hydrochloric acid. as observed by O’Leary and Papish ( 1 2 ) . T o carry out the precipitation, the acid was added to a centrifuge tube containing a definite volume of standard cesium solution t o make the total VOL. 29, NO. 8, AUGUST 1957
1 181
volume 5 ml. T o this solution was added 1 ml. of the tungstosilicic acid solution. The contents were mixed b y bubbling air into the tube. The time required for quantitative precipitation was approximately 35 minutes.
EFFECTOF ACIDITY. As the acidity of the precipitation medium increases, the solubility of the cesium tungstosilicate decreases. I n conjunction with this, the rate of precipitation also increases. T o illustrate this, samples containing 0.133 mg. per ml. of cesium were prepared by adding 0.50 ml. of standard cesium solution to each 15-ml. test tube containing the following amounts of 6.V hydrochloric acid: 1.15, 2.58, 3.66, and 4.50 nil. Distilled water was added to make the total volume of each solution 5.0 nil. These solutions vere 2, 4, 5, and 6 S , respectively. in hydrochloric acid. T o each of these solutions was added 1.0 ml. of the tungstosilicic acid solution. The solutions were then mixed and observed a t definite time intervals. The significance of acidity is shown in Table I. The type of acid used has a significant effect on the precipitation. O’Leary and Papish ( 1 1 ) stated that the precipitation of cesium is much more rapid in 8 9 sulfuric acid, but the separation from rubidium is not as sharp as with hydrochloric acid; nitric acid prevents precipitation of both cesium and rubidium tungstosilicate. EFFECT OF TIME. The completeness of precipitation was checked with respect to time. Low values for cesium were obtained when less than 30 minutes were allowed for the precipitation. This was interpreted as being due to incomplete precipitation, as more than 30 minutes gave satisfactory results. The optimum time was 35 minutes. Color System. For the reduction of tungstosilicic acid to the heteropoly blue color, titanium trichloride was selected (12). A solution of tungstosilicic acid n a s adjusted to approximately p H 0.2. To this solution was added 8 nil. of titanium trichloride reagent. The blue color developed immediately. As the heteropoly blue color had an absorption peak a t 725 mp (see Figure 1). all measurements were made at that m-ave lmgth. The color began to fade appreciably after standing for approximately 1 hour exposed to the air. However. if the solution n a s placed in an inert atmosphere immediately after the color
Table
2
Clear Clear Clear Clear Clear
1 182
I.
I
0‘ 500
WAVE LENGTH, mu
Figure 1 . Absorption spectra of tungsten blue and heteropoly blue 1.
2.
Reduced tungstic ocid solution Reduced tungstosilicic ocid solution
developed, the color was stable for a t least 10 days. The instability of the color ivas attributed to air oxidation of the colored reduced species back to the original form, tungstosilicic acid. As the color is sufficiently stable in air, there was no need to store it in an inert atmosphere. The term heteropoly blue is used for the product obtained from tungstosilicic acid, and tungsten blue, for that obtained by reduction of tungstic acid ( 2 ) . Both curves are shown in Figure 1. EFFECTOF ACIDITY. T o establish t h e most desirable acidity, samples containing 0.80 ml. of tungstosilicic acid reagent vere diluted t o 5.00 ml. with 6&Irhydrochloric acid. This acidity should simulate t h a t of t h e supernatant liquid obtained after the precipitation reaction. These samples were placed in a 25-nil. volumetric flask and treated with varying amounts of 3N sodium hydroxide solution. After the solution had cooled, 8 nil. of titanium trichloride reagent was added and the resultant blue color was measured. TT’ith less than 7 ml. of sodium hydroxide solution the resultant color was instable and not intense (see Figure 2). K i t h more than 7 ml. of base a stable color system was obtained. When more than 10 ml. of base was used, the solution was basic and the reductant precipitated. Eight milliliters was selected as the optimum amount of base required. The acidity after treatment with 8 ml. of base was approximately p H 0.2. The p H of the resultant colored solution when diluted to volume 11-as appro-& mately 0.7. The intensity of the color system was constant from pH 0.1 t o 2.1. COKFORMITY TO BEER’SLaw. T h e
Effect of Acidity on Precipitation Reaction
Acidity of Hydrochloric Acid Solvent, 3 4 5 Clear Clear Clear Clear Clear Ppt. Clear Ppt. Clear Ppt.
ANALYTICAL CHEMISTRY
700
600
6
Time, hlin.
Clear Ppt. Ppt. Ppt. Ppt.
15 30 60 90 120
085
I
I
1
I
1
~
adherence t o Beer’s law of t h e overall procedure, including the precipitation and color reaction, was checked. For samples of stock cesium solution run according t o the following recommended procedure the absorbance varied linearly with concentration. The relationship, hon ever, differs from the usual type in t h a t the intensity varies inversely with the concentration of cesium. Conformance t o R e d s law was obtained for concentrations of cesium ranging from 0.133 to 0.931 mg. per ml. Above 0.931 mg. there is a slight deviation from linearity. Below 0.133 mg. the solubility of the salt causes deviation. EFFECT OF DIVERSE IONS. The procedure for t h e study of interferences \vas t o dissolve 25 Ing. of t h e solid yielding the diverse ion in 5.00-ml. samples containing 0.399 mg. per ml. of cesium. T h e samples were prepared by adding 1.50 ml. of cesium stock solution and 3.50 nil. of 6.1- hydrochloric acid solution t o a 15-ml: centrifuge tube. T h e slight changes of volume due to dissolution of the solid were neglected. Ions tested for interference are listed in Table I1 together with the formulas of the solids
used to provide them. Then the steps given in the recommended procedure were followed. All tests were run in cluplicate. The interference or noninterference of the diverse ion was established by reference to control limits. These limits mere calculated from observed absorbances of 12 samples containing no extraneous ions. The data are given in Table 111. If the range of the duplicate pair of samplcs exceeded 0.030, these values TI ere disregarded and another pair n a s prepared and measured. If thr nicnn akmrbance of t h c p a i r was outside the control limits for X , the concentration of the interfering ion was lowered until the mean absorbance was within the control limits. A critical study was madc of the interftwnce effects of the alkali metal and ariinioriiuni ion>, a? these constituents are commonly found in the presence of cesium anti usually intt>rferen4th its determination. Lithium and sodium do not interfcrc; however, ammonium. potassium, anti rubidium must not be present in exress of the specified amounts, as they may also precipitate with tungstosiiicir acid, thus causing high results. Oxidizing agents interfere n i t h the color reaction. causing a marked decrease in the intenqity of the color. These ions seriouqly interfere even when additional reductant ib added. Copper, iron, and aluminum have a bleaching effect upon the color system. The colorcd ions of cobalt and nickel can be toleratrd within the specified amounts. Fluoride interferes even in the presence of boric acid. Evidence intlicatcti that the fluoroborate ion also interfrrcs n ith thc color reaction. The maximum aniounts of the ions studied which may he present without interference arc given in Tnlile 11. RECOMMENDED PROCEDUke
Calibration Curve. Dispense from a microburet 0.50, 1.00, 2.00. 2.50. and 3.50 nil. of t h e standard cc3siuni solution into 15-ml. centrifuge tubes. T o each solution add 6-I‘hydrochloric acid t o make t h e total volume in cach t u b e exactly 5.00 ml. T h e samples should contain 0.133, 0.266, 0.532, 0.665, and 0.931 mg. per nil. of cesium. Add s l o ~ d y 1.00 ml. of 5% tungstosilicic acid rcagcnt. Shake t’hc Centrifuge tulle con$tnntly during thc addition. Stir the solut,ioii by bubbling air into thc tube for 2 minutes. Allon. the solution to stand for 35 minutes, after which centrifuge for 3 minutes. By nieans of a pipet transfcr 5.00 ml. of the supernatant liquid containing the exccss unreactcd tungstosilicic acid into a 25-in1. volumet’ric flask. T o this solution s l o d y add 8 ml. of 3 5 sodium hydroxide n.hile constantly shaking the flask. The pH should be approxiniately 0.2. Develop the heteropoly blue color by adding 8 ml. of 0.2% titanium trichloride reagent and dilute
Table II.
Suspected Interfering Ions .il-++
BO,--Br-
C?HqO,Ca++ C1Co++
cu
+T
C12O;--
Fe++F-
IKT Li IIg
+
+
AIn04M O O 4 -SH, ?;aSi-+
SO1S O ?Po?--Rh Sr+SO4 - -
Table 111. Statistical Study o f Reproducibility of Measurement of Cesium“
Effect of Interfering Ions
Compounds .%dded Ala(S04), &BO,
SaBr
SaC?H302
CaCL
SaCl CoC12 C 11C1,J Ii2cr;o.i
FeCl, SaF SaI KCl LiCl MgSO4 IiL\In04 r\-a?AIoOl
Max. Permissible Amt. for Adherence to Control Limits, RIg. 0
25 25 25 25 25
10 1 0 0 1
25 7.50 25 25 0 1 1
SHdCl YaC1 SiS0,
25 5
SaS03 S3SO2
1 1
SaH11-’04 RhC1 SrCl? Sa?SOn
20 0.56 25 25
to volume rvith distilled water. The color develops immediately. Transfer some of the solution to a 1-cm. cell and measure the absorbance at 725 mw. Use distilled water as a blank in the reference cell. Construct a curve with absorbance as the ordinate and concentntion as the abscissa. Procedure. By means appropriate t o t h e cesium-bearing material, prepare a solution approximately 6 N in hydrochloiic acid. Dilute, if necessary, with 6 S hydrochloric acid in a volunietric flask t o a volume such t h a t t h e concentration ~villbe in t h e range of 0.1 t o 0.9 mg. per ml. of cesium. Transfer a 5-ml. aliquot of the prepared solution t o a 15-ml. centrifuge tube. Develop the color as described for constructing the calibration curve. Measure the absorbance and read from the calibration curve the amount of cesium. If the concentrations of interfering ions exceed the amounts permissible (see Table 11),erroneous rcsults nil1 be obtained unlcss suitable separative trfatnient can be devised. PRECIYIOS ~ S DXCCCRACY.T n elve samples containing 0.399 nig. of cesium per nil. were run according to the recomniendcd procedure. The results of the reproduciliility tests are shon n in Table 111. The qtnndrtrrl deviation wa.: calculated to tie 0.00767. which when t r a n s l a t d to per cent of cebium, allows 11.91yo error. The range of the measured absorbances TI as 0.030. Sample. of varying cesium concentrations within the calibration limit were run to test the accuracy of the proposed method. T o definite amounts of stock cesium solution in 15-nil. centrifuge
X = individual values observed n = 12 = number of values observed u
=
dev. _ -- 0.00819 = estimate of true std. cz dev. of population of which measured values of X belong CI = factor relating an observed u for a given n to its U ’ x = 0.7815 = av. of observed values y = X‘ = estimate of true mean of population t o which measured values of X belong R = range of samples = highest X value minus lox-est X value Using 99y0 levels for calculating control limits for determinations made in duplicate: Lower control limit for R = 0 . u ’ = 0 Upper control limit for R = 3.69 . u’ = 0 030 Loner control limit for B = 8’- 2.12 u ’ = 0.764 Upper control limit for 2 = 2’ 2.12 u ’ = 0.799 a For explanation of symbols and methods, see ( 5 ) . q’
=
+
tubes was added 6S hydrochloric acid to make the total volume of the sample 5.00 nil. These 5.00-ml. samples were run as described in the recommended procedure. The results of this study are shown in Table IT’.
Table IV.
Determinaf;on o f Cesium in Synthetic Samples
Cesium, hIg./hIl. Taken Found 0 123 0 133 0 125 0 133 0 266 0 266 0 399 0 405 0 392 0 399 0 532 0 535 0.798 0 792 0 945 0 931
Relative Error, c /o
-7 -6 0 4 1
51 01 00 50
-1 75 +O 56 -0 75 + l 50
CONCLUSIONS
The procedure described offers a convenient, accurate, and reasonably selective method for determining milligram quantities of cesium. Potassium and rubidium can be tolerated nithin limited amounts. Inasmuch as washing and further handling of the precipitate are eliminated, the method is rapid. Once the calibration curve is cstablished mid the samples 3 r prepared, ~ as many as five dcterrninations may be run n ithin a n hour. The method has relatively low sensitivity and there are a number of interferences n i t h the color reaction. ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support of this work by VOL. 2 9 , NO. 8, AUGUST 1957
1183
means of a fellowship grant b y Eli Lilly and Co. LITERATURE CITED
(1) Bassett, L. G., Byerly,
W.,“Sodium,
Potassium, Rubidium, and Cesium,’’ in “Analytical Chemistry of the Manhattan Proiect.” ed..~. ~~~~. ited by C. J. Rodden; ~ p p’345-8, . bfcGraw-Hill, Kerv York, 1950. (2) Boltz, D. F., Mellon, M. G., Ax.4~. CHEM.19, 873-84 (1947). ~~
~~~
~
(3) Burkser, E. S., Feldman, R. V., Zavodskaya Lab. 7, 166-8 (1938). (4) Burkser, E. S., Kutschment, M. S., Mikrochemie 18, 18-21 (1935). (5) Burr, I. W.,“Engineerivg Statistics
16) (7) (8)
(9)
and Quality Control, LIcGrawHill, Yew York, 1953. Duvsl. C.. Chim. anal. 34, 209-21 (1952). Geilman, W,, Gebauhr, W.,Z. anal. Chem. 142, 241-54 (1954). Godeffroy, R., Ber. 9, 1363-8 (1876). Kahn, B., ANAL. CHEX 28, 216-18 (1956).
(10) Moser, L., Rietschel, E., ,l.ionalsh. 46, 9-22 (1925). (11) O’Leary, W. J., Papish, J., IND. ENG.CHEM.,ANAL.ED.6, 107-11 (1934). (12) Scroggie, A., J . Ana. Chem. SOC. 51, 923-5 (1929). (13) Snell, F. D., Snell, C. T., “Colorimetric hIethods of Analysis,” Vol. IV. v . 482. Van Nostrand. New York’, 1954.’ RECEIVEDfor reviex September 13, 1956. hccepted March 25, 1957.
Determination of Salicylic Acid in Aspirin CLIFTON W. STRODE, Jr.1, F. N. STEWART, H. 0. SCHOTT, and 0. J. COLEMAN F. Queeny Plant, Monsanto Chemical Co.,St. Louis, Mo.
John
b Spectrophotometric and visual techniques for the quantification of the U.S.P. limit test for salicylic acid in acetylsalicylic acid and aspirin tablets are described. By controlling the variables which may affect the ferric salicylate color and the hydrolysis of aspirin, precision and accuracy within 0.005% are illustrated.
T
ESTS LISTED I ~ Y the U. S. Pharmacopeia for salicylic acid in acetylsalicylic acid and aspirin tablets ( 7 ) do not fulfill the needs of modern industry because they are limit tests-Le., they only demonstrate that the material being examined contains more or less than an apparent 0.10 or 0.15% free salicylic acid. hfuch of the acetylsalicylic acid produced today contains less than 0.05% free salicylic acid, and i t is frequently necessary that the manufacturer know within 0.00570 or less the exact salicylic acid content of his product. Consequently, many unpublished modifications and extensions of the U.S.P. test have evolved, such as increasing sample and alcohol concentrations, using other solvents, and titrating a ferric alum blank with standard salicylic acid to match the color of the sample solution. As these changes have been developed independently, and in some cases apparently without proper consideration of the variables involved, conflicting data are frequently encountered. Edwards and coworkers (3) have made a comprehensive study of these variables, including the kinetics of aspirin hydrolysis and the effects of factors such as pH and ionic strength on the ferric salicylate test specified
Deceased.
1 184
ANALYTICAL CHEMISTRY
in the British Pharmacopoeia. They recommend conducting the test at 25” C. in a solution buffered a t p H 2.9. Colorimetric measurements are made a t timed intervals to provide an extrapolated correction for the hydrolysis of aspirin to salicylic acid. A simplified procedure is suggested involving a single measurement corrected for the previously determined hydrolysis rate a t a fixed temperature and weight of sample. Parallel work in this laboratory involving modification of the U.S.P. test, which specifies a more dilute solution of aspirin than does the B.P. test, has essentially verified the conclusions of EdJvards. The hydrolysis of aspirin must be controlled by definition of procedural details involving concentration, temperature, pH. and the time during Jvhich hydrolysis is significant. Calibration or comparison standards should be a t the same p H and essentially of the same composition as the sample solutions. il. practical test should provide a depth of solution commensurate with adequate sensitivity. Incorporating these considerations, the following spectrophotometric method has been in routine use in this laboratory for the past 12 years. The accompanying visual comparison method has been used in routine process control for the past 2 years. SPECTROPHOTOMETRIC METHOD
This method is somewhat analogous to the simplified procedure suggested by Edwards in that a previously estimated hydrolysis correction is eniployed, It is applicable for the determination of up to about 0.25% free salicylic acid in crystalline acetyl-
salicylic acid or in granular or tableted starch-aspirin formulations nhich may be white or tinted with pink or green dyes. Apparatus and Reagents. A Photovolt Model 102-E Lumetron colorimeter n-ith monochromatic filters a n d 100-mm. cylindrical cells is used for transmittance measurements. Other instruments with similar light paths t h a t transmit relatively narrow bands a t about 515 and 575 mM would probably be suitable. Acetylsalicylic acid is purified by recrystallization from isopropyl alcohol to an apparent free salicylic acid content of less than 0.005%. Ferric alum solution is prepared by dissolving 4.0 grams of ferric ammonium sulfate in 40 ml. of water and diluting to 50 ml. X 10-ml. aliquot of this solution and 10 ml. of 0.5S hydrochloric acid are diluted to 500 nil. with xater. The dilute solution is stored in a refrigerator and freshly prepared each
week. Standard salicylic acid is prepared by dissolving an accurately weighed 0.1-gram portion of sublimed salicylic acid in 10 ml. of specially denatured Formula 30 alcohol (SD 30, 1 volume of methanol in 10 volumes of 190 proof ethyl alcohol), adding 0.1ml. of glacial acetic acid, and diluting to 1 liter with water. Calibration. I n oidei t o minimize t h e time during which hydrolysis takes place, the colorimeter, after a 10-minute warm-up period, is balanced t o 100% transmittance a t 515 mp with a blank. The blank is 1.9 ml. of SD 30 alcohol, 0.5 ml. of glacial acetic acid, and 2.0 ml. of dilute ferric alum solution, diluted to 100 ml. with Ivater. It should be freshly prepared. From 0.2- to 2.0-ml. aliquots of the standard salicylic acid solution, equivalent to O . O l ~ o to O.lyOsalicylic acid in 0.2-gram samples of aspirin, are
